| Gas diffusion in porous media is a key research problem in the fields of energy exploitation,design and manufacture of porous materials,chemical engineering and so on.It is of great significance to explore the gas diffusion law in the porous media and construct the gas diffusion model of the porous media for the exploitation and evaluation of unconventional natural gas and the design and manufacture of the gas diffusion layer of the fuel cell.However,due to the complex structure,variety and great difference between different porous media,it is difficult to comprehensively analyze the influence of pore microstructure on gas diffusion in porous media,which makes the traditional research methods have great limitations in the study of gas diffusion in porous media.A large number of studies have shown that most porous media have self-similarity and random fractal characteristics.Therefore,in this paper,the internal structure of porous media is simulated by fractal theory,and the pore structure of complex porous media is qualitatively and quantitatively described.Then the diffusion law of gas in porous media with fractal structure is systematically studied and the mathematical model of diffusion process in porous media is established.The main research contents are as follows:(1)The relevant parameters in the curved capillary bundle model with fractal characteristics are analyzed,and then the flow region in the capillary bundle model is divided.According to the divided flow region,the permeability correction factor is simplified,and the flow rate model in the single curved capillary is obtained which is easy to calculate.(2)Based on the curved capillary bundle model and the simplified permeability correction factor model,combined with Fick’s law and the diffusion law of single-phase gas,the effective diffusion coefficient of single-phase gas in the new porous media was deduced.The accuracy of the new theory is verified by comparing with the experiment,and the influence of the parameters such as porosity,surface integral dimension and tortuosity fractal dimension on the effective diffusion coefficient is analyzed.(3)Based on the curved capillary bundle model and gas-liquid two-phase flow characteristics,the mathematical model of the effective diffusion coefficient of gas in the unsaturated gas diffusion layer is established.According to the different surface characteristics of porous media,it is divided into two types: hydrophilic and hydrophobic,and compared with the experimental data respectively,which proves the correctness of the gas-liquid two-phase diffusion model.The effects of hydrophilicity and hydrophobicity on diffusion were analyzed in detail.(4)The fractal-Monte Carlo method is used to further demonstrate the mathematical model of the gas effective diffusion coefficient established in this paper.The fractal-Monte Carlo expression is carried out for the single-phase gas diffusion model and the gas-liquid two-phase diffusion model.The effective diffusion coefficients in single-phase and two-phase cases are output by computer program.The correctness of the numerical simulation results is verified by comparing with experimental data and mathematical model.Finally,the key parameters in the numerical simulation process are analyzed.The results show that permeability correction factor,porosity,fractal dimension,maximum pore diameter and other parameters have a great influence on the effective diffusion coefficient in single-phase gas diffusion.In the process of gas-liquid two-phase diffusion,hydrophilicity,hydrophobicity,liquid saturation,fractal dimension and other parameters have a great influence on the diffusion coefficient.Fractal-Monte Carlo method has strong applicability in the study of gas diffusion in porous media.The above work has a certain reference value for the evaluation and development of unconventional natural gas and the design and manufacture of fuel cell gas diffusion layer. |
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